The processes of proliferation, commitment and differentiation of mesenchymal cells will be studied at the cellular level using the teratoma-derived, bipotential cell line 1246 and an insulin- independent variant cell line derived from 1246 cells as a model system. The 1246 cell line has the following properties; (1) it can differantiate either into adipocytes or into muscle cells; (2) it can proliferate and undergo adipose differentiation in defined medium whereas other adipogenic cell lines can grow in defined medium but require fetal calf serum to differentiate; 3) its growth and differentiation are stringently dependent on the presence of insulin; and 4) its conditioned medium is able to replace serum to stimulate the differentiation of several adipogenic cell lines. From 1246 cells, an insulin-independent variant cell line has been isolated. This cell line which has lost the ability to differentiate produces in its culture medium an adipose differentiation inhibitor active to block differentiation of several adipogenic cell lines and of primary cultures of rat and mouse adipocytes. Experiments are proposed to characterize, purify and study the expression of both types of adipose differentiation regulators produced by both cell lines. This study will be part of the long-term objective of our investigations related to the characterization of factors controlling growth, commitment and differentiation processes of mesenchymal-derived cell lines and to understand their mechanism of action.